Digital X-ray detectors (DXDs) contain a two-dimensional array of pixel elements. Each pixel element typically contains a diode element and a thin film transistor (TFT) element. The diode element collects light that is converted from Xrays incident on a scintillator material. The TFT element acts as a switch. When the switch is turned “OFF”, no charge is transferred from the diode element onto the readout circuitry via the data line. When the switch is turned “ON”, any charge collected on the diode element is transferred onto readout circuitry via the data line. The TFT element configuration is controlled by an applied gate voltage. In the case of DXD detectors the applied gate voltage is a ROW based operation where all pixels in a given ROW have a common row gate control line.
FIG. 1 is a perspective view of a prior art digital radiographic (DR) imaging system 10 that includes a generally planar DXD 40 (shown without a housing for clarity of description), an x-ray source 14 configured to generate radiographic energy (x-ray radiation), and a digital monitor 26 configured to display images captured by the DXD 40, according to one embodiment. The DXD 40 may include a two dimensional array 12 of detector cells 22 (photodiodes), arranged in electronically addressable rows and columns. The DXD 40 may be positioned to receive x-rays 16 passing through a subject 20 during a radiographic energy exposure, or radiographic energy pulse, emitted by the x-ray source 14. As shown in FIG. 1, the radiographic imaging system 10 may use an x-ray source 14 that emits collimated x-rays 16, e.g. an x-ray beam, selectively aimed at and passing through a preselected region 18 of the subject 20. The x-ray beam 16 may be attenuated by varying degrees along its plurality of rays according to the internal structure of the subject 20, which attenuated rays are detected by the array 12 of photosensitive detector cells 22. The planar DXD 40 is positioned, as much as possible, in a perpendicular relation to a substantially central ray 17 of the plurality of rays 16 emitted by the x-ray source 14. The array 12 of individual photosensitive cells (pixels) 22 may be electronically addressed (scanned) by their position according to column and row. As used herein, the terms “column” and “row” refer to the vertical and horizontal arrangement of the photosensor cells 22 and, for clarity of description, it will be assumed that the rows extend horizontally and the columns extend vertically. However, the orientation of the columns and rows is arbitrary and does not limit the scope of any embodiments disclosed herein. Furthermore, the term “subject” may be illustrated as a human patient in the description of FIG. 1, however, a subject of a DR imaging system, as the term is used herein, may be a human, an animal, an inanimate object, or a portion thereof.
In one exemplary embodiment, the rows of photosensitive cells 22 may be scanned one or more at a time by electronic scanning circuit 28 so that the exposure data from the array 12 may be transmitted to electronic read-out circuit 30. Each photosensitive cell 22 may independently store a charge proportional to an intensity, or energy level, of the attenuated radiographic radiation, or x-rays, received and absorbed in the cell. Thus, each photosensitive cell, when read-out, provides information defining a pixel of a radiographic image 24, e.g. a brightness level or an amount of energy absorbed by the pixel, that may be digitally decoded by image processing electronics 34 and transmitted to be displayed by the digital monitor 26 for viewing by a user. An electronic bias circuit 32 is electrically connected to the two-dimensional detector array 12 to provide a bias voltage to each of the photosensitive cells 22.
Each of the bias circuit 32, the scanning circuit 28, and the read-out circuit 30, may communicate with an acquisition control and image processing unit 34 over a connected cable (wired) 33, or the DR detector may be equipped with a wireless transmitter to transmit radiographic image data wirelessly 35 to the acquisition control and image processing unit 34. The bias circuit 32, the scanning circuit 28, and the read-out circuit 30 may be formed as electronic integrated circuits for readout (ROICs). The acquisition control and image processing unit 34 may include a processor and electronic memory (not shown) to control operations of the DXD 40 as described herein, including control of ROICs 28, 30, and 32, for example, by use of programmed instructions. The acquisition control and image processing unit 34 may also be used to control activation of the x-ray source 14 during a radiographic exposure, controlling an x-ray tube electric current magnitude, and thus the fluence of x-rays in x-ray beam 16, and/or the x-ray tube voltage, and thus the energy level of the x-rays in x-ray beam 16.
The acquisition control and image processing unit 34 may transmit image (pixel) data to the monitor 26, based on the radiographic exposure data received from the array 12 of photosensitive cells 22. Alternatively, acquisition control and image processing unit 34 can process the image data and store it, or it may store raw unprocessed image data, in local or remotely accessible memory.
With regard to a direct detection embodiment of DXD 40, the photosensitive cells 22 may each include a sensing element sensitive to x-rays, i.e. it absorbs x-rays and generates an amount of charge carriers in proportion to a magnitude of the absorbed x-ray energy. A switching element may be configured to be selectively activated to read out the charge level of a corresponding x-ray sensing element. With regard to an indirect detection embodiment of DXD 40, photosensitive cells 22 may each include a sensing element sensitive to light rays in the visible spectrum, i.e. it absorbs light rays and generates an amount of charge carriers in proportion to a magnitude of the absorbed light energy, and a switching element that is selectively activated to read the charge level of the corresponding sensing element. A scintillator, or wavelength converter, is disposed over the light sensitive sensing elements to convert incident x-ray radiographic energy to visible light energy.
Examples of sensing elements used in sensing array 12 include various types of photoelectric conversion devices (e.g., photosensors) such as photodiodes (P-N or PIN diodes), photo-capacitors (MIS), photo-transistors or photoconductors. Examples of switching elements used for signal read-out include MOS transistors, bipolar transistors and other p-n junction components.
FIG. 2A is a schematic diagram 240 of a portion of a two-dimensional array 12 for a DXD 40. The array of photosensor cells 212, whose operation may be consistent with the photosensor array 12 described above, may include a number of amorphous silicon (a-Si) or hydrogenated amorphous silicon (a-Si:H) n-i-p photodiodes 270 and thin film transistors (TFTs) 271 formed as field effect transistors (FETs) each having gate (G), source (S), and drain (D) terminals. In embodiments of DXD 40 disclosed herein, the two-dimensional array of photosensor cells 12 may be formed in a device layer that abuts adjacent layers of the DR detector structure. A plurality of gate driver circuits 228 (ROICs) may be electrically connected to a plurality of gate lines 283 which control a voltage applied to the gates of TFTs 271, a plurality of readout circuits 230 (ROICs) may be electrically connected to data lines 284, and a plurality of bias lines 285 may be electrically connected to a bias line bus or a variable bias reference voltage line 232 which controls a voltage applied to the photodiodes 270. Charge amplifiers 286 may be electrically connected to the data lines 284 to receive signals therefrom. Outputs from the charge amplifiers 286 may be electrically connected to a multiplexer 287, such as an analog multiplexer, then to an analog-to-digital converter (ADC) 288, or they may be directly electrically connected to the ADC, to stream out the digital radiographic image data at desired rates. In one embodiment, the schematic diagram of FIG. 2 may represent a portion of a DXD 40 such as an a-Si based indirect flat panel imager as described below.
Incident x-rays, or x-ray photons, 16 are converted to optical photons, or light rays, by a scintillator, which light rays are subsequently converted to electron-hole pairs, or charges, upon impacting the a-Si n-i-p photodiodes 270. In one embodiment, an exemplary detector cell 222, which may be equivalently referred to herein as a pixel, may include a photodiode 270 having its anode electrically connected to a bias line 285 and its cathode electrically connected to the drain (D) of TFT 271. The bias reference voltage line 232 can control a bias voltage of the photodiodes 270 at each of the detector cells 222. The charge capacity of each of the photodiodes 270 is a function of its bias voltage and its capacitance. In general, a reverse bias voltage, e.g. a negative voltage, may be applied to the bias lines 285 to create an electric field (and hence a depletion region) across the pn junction of each of the photodiodes 270 to enhance its collection efficiency for the charges generated by incident light rays. The image signal represented by the array of photosensor cells 212 may be integrated by the photodiodes while their associated TFTs 271 are held in a non-conducting (off) state, for example, by maintaining the gate lines 283 at a negative voltage via the gate driver circuits 228. The photosensor cell array 212 may be read out by sequentially switching rows of the TFTs 271 to a conducting (on) state by means of the gate driver circuits 228. When a row of the pixels 22 is switched to a conducting state, for example by applying a positive voltage to the corresponding gate line 283, collected charge from the photodiode in those pixels may be transferred along data lines 284 and integrated by the external charge amplifier circuits 286. The row may then be switched back to a non-conducting state, and the process is repeated for each row until the entire array of photosensor cells 212 has been read out. The integrated signal outputs are transferred from the external charge amplifiers 286 to an analog-to-digital converter (ADC) 288 using a parallel-to-serial converter, such as multiplexer 287, which together comprise read-out circuit 230.
This digital image information may be subsequently processed by image processing system 34 to yield a digital image which may then be digitally stored and immediately displayed on monitor 26, or it may be displayed at a later time by accessing the digital electronic memory containing the stored image. The flat panel DXD 40 having an imaging array as described with reference to FIG. 2 may be capable of both single-shot (e.g., static, radiographic) and continuous (e.g., fluoroscopic) image acquisition. Moreover, much of the control electronics in the image processing system 34 may be contained with a housing of the DXD panel 40.
FIG. 2B shows an example schematic of a single pixel structure with one photodiode element, and one TFT element controlled by a single gate line control per physical row, illustrating exemplary voltage control levels for the gate line (row) of about 20 V (“on”) to about −4 V (“off”). For a given pixel size the majority of the area is taken by the photodiode element, as illustrated in the area representation of an imaging pixel of FIG. 2C. Some of the area may be taken by the TFT element. The TFT size may be as small as possible to maximize the area of the photodiode element. An indium gallium zinc oxide (IGZO) TFT element due to its higher mobility can be made smaller than an amorphous silicon (a-Si) TFT.
FIG. 3 shows an example two-dimensional array area layout of the single unit pixel of FIG. 2C. The number of data line (readout) output lines 302 in the array will equal the number of physical columns. The number of gate lines 304 in the array will equal the number of physical rows. Also, the number of TFT elements on the output data line is equal to the number of rows. For a large DXD detector there may be many ROIC assemblies required to readout the entire panel. In a high performance panel and a dual sided readout configuration a second set of ROIC assemblies is a common architecture. For low cost detector DXD applications, it would be advantageous to reduce the number of required ROIC assemblies to reduce DXD panel cost.
One way to reduce the number of external ROICs is illustrated in FIG. 4 which shows a common multiplexing architecture where two data lines from the pixel array are multiplexed down to one physical data line output using a multiplexing circuit (“mux”) at the array end. This is a common architecture of an array of single pixels and a multiplexing output data line placed at one end of the array. In this configuration two separate and adjacent pixel data lines can be reduced to one, which may result in a 2× reduction in the number of ROICs. In order to successfully read out all pixels using one shared data line, typically it would take two line readout times in order to readout one physical row. One method of using the structure of FIG. 4 is to combine the detected photodiode charges in horizontally adjacent pixels that share one output data line, referred to as binning or, more specifically, horizontal 2× binning. This multiplexing scheme does not provide a signal-to-noise (SNR) advantage with respect to horizontal charge binning due to the fact that charge is shared on two separate data lines (prior to the mux) and data line noise is the dominant noise source in a typical DXD panel.